Tachometer For Tires

ABSTRACT

A device for counting the rotations of a tire on a vehicle using the deformations that the tire undergoes on each rotation of the wheel, comprising a casing, a sensor intended to generate an electrical signal on each rotation of the wheel, and connection means intended to mechanically connect the casing and the sensor to the surface of the tire. The sensor is in the form of a passive sensor in one piece with the casing, whereof the signals emitted are in proportion with the variations over time in a magnetic flux through the said passive sensor, and a magnetic field generator is arranged opposite the passive sensor. The connection means mechanically connect the casing to a first surface A and the magnetic field generator to a second surface B, the two surfaces A and B being substantially coplanar and offset from one another in a direction D by a given distance L. The electrical signal is produced by the relative displacements of the said passive sensor and the said magnetic field generator.

The present invention relates to a device intended to be carried by atire, for detecting the revolutions thereof as it travels. It alsorelates to a tire carrying a device of this kind.

An automotive vehicle generally has an odometer allowing the totaldistance that this vehicle has covered to be known. However, thiscounter provides no information on the distance covered by each tire ofthe vehicle.

For example, when tires are changed temporarily (for example in winter,when snow tires are put on) or when old worn-out tires are replaced, thedistance displayed by the odometer no longer indicates the distanceactually covered by the tires.

Conventionally, the only indication of the distance covered by a worntire is thus the wear to its tread.

Tire manufacturers have thus had the idea of inserting a device which isone piece with the tire and inside the latter and which indicates thedistance covered by this tire. Such devices already exist.

Of these devices, U.S. Pat. No. 5,877,679 discloses a device forcounting the rotations of a tire on a vehicle, comprising a rigidcasing, connection means intended to connect the casing mechanically tothe inner surface of the tire, and a sensor intended to generate anelectrical signal on each rotation of the wheel, in which the connectionmeans comprise at least two mounting blocks which are offset from oneanother and the sensor is a force sensor arranged between the twomounting blocks.

The existing devices have a limited service life, however, since theyhave a high level of power consumption and are complex.

The invention relates to a similar device in which the sensor is in theform of a magnetic field generator and a sensor sensitive to themagnetic field and in one piece with the casing, arranged opposite thepassive sensor, in that the connection means mechanically connect thecasing to a first surface A and the magnetic field generator to a secondsurface B, the two surfaces A and B being substantially coplanar andoffset from one another in a direction D by a given distance L, and inthat the said electrical signal is produced by the relativedisplacements of the said passive sensor and the said magnetic fieldgenerator.

Preferably, the sensor sensitive to the magnetic field is a passivesensor.

This device is simpler than the existing devices and so more robust.

The sensor of the device, unlike the force sensor of the prior art, hasno need of power in order to work. This allows the device of theinvention to have greater autonomy than existing devices.

Optionally, the passive sensor of the device according to the inventionmay be a simple loop of conductive wire. In this case, electricaltension is created in the loop by induction when it is subjected to avariation in magnetic flux over time. One advantage of this coil is itsgreat robustness. Unlike the force sensors or other sensors usedpreviously in the prior art, which are complex and fragile sensors, asimple coil of conductive wire has a high resistance to the vibrationscreated by travel over the ground.

According to a first preferred embodiment, the connection means are suchthat a relative displacement normal to the direction D of the surfaces Aand B brings about a relative displacement of the magnetic fieldgenerator and the sensor sensitive to the magnetic field substantiallynormal to the direction D.

The magnetic field generator and the sensor sensitive to the magneticfield may be arranged in opposition in a direction normal to thedirection D.

According to a first example embodiment, the connection means, thecasing and the sensor sensitive to the magnetic field may advantageouslybe arranged in an L shape.

The means for connecting the casing and the sensor may also comprise aflexible soleplate having a first face intended to be permanentlyconnected to the surface of the tire and at least one connection elementto which the casing is fixed at a given spacing in relation to thesoleplate.

The magnetic field generator is in this case advantageously embedded inthe soleplate. Preferably, the casing, the element for connecting thecasing and the soleplate are generally U-shaped.

According to a second example embodiment, the means for connecting themagnetic field generator are in the form of a resilient connectionelement of arcuate elongate shape extending in the direction D betweenthe surface B and a zone of the casing or element for connecting thecasing which is opposite the surface A, the magnetic field generator andthe sensor sensitive to the magnetic field being arranged in oppositionbetween the two ends of the said element for connecting the sensor.

Preferably, the connection means comprise a flexible soleplateconnecting the surfaces A and B.

This device has the advantage of allowing the surfaces A and B to beseparated from one another and thus to increase the amplitude ofrelative movements between the two elements of the sensor.

According to a third example embodiment, the means for connecting themagnetic field generator are in the form of a resilient connectionelement of arcuate elongate shape extending in the direction D betweenthe surface B and a surface B′ arranged beyond the surface A, themagnetic field generator and the sensor sensitive to the magnetic fieldbeing arranged in opposition between the two ends of the said elementfor connecting the magnetic field generator.

Preferably, the connection means comprise a flexible soleplateconnecting the surfaces A, B and B′.

This device makes it possible in certain conditions, for example when itis arranged on the inner surface of the tire below the crown, to makethe device for counting the rotations of the tire more sensitive. Thedevice is in this case preferably arranged with the direction D orientedin the circumferential direction. It is also possible to arrange thisdevice on the surface of the sidewall, with the orientation still in thecircumferential direction.

The magnetic field generator and the sensor sensitive to the magneticfield may also be arranged in opposition in a direction parallel to thedirection D.

According to a fourth example embodiment, the connection means comprisean element for connecting the casing separating the said casing from thesurface A by a given height and an element connecting the magnetic fieldgenerator separating the said magnetic field generator from the surfaceB by a given height, and in which the sensor sensitive to the magneticfield is in one piece with the casing and is arranged at a height inrelation to the surface A which is substantially identical to the heightseparating the magnetic field generator from the surface B.

A device of this kind may preferably be arranged close to the beads ofthe tire with a radial orientation D.

According to a second embodiment, the connection means are such that arelative displacement normal to the direction D of the surfaces A and Bbrings about a relative displacement of the sensor sensitive to themagnetic field and the magnetic field generator substantially parallelto the direction D.

According to a fifth example embodiment which corresponds to this secondembodiment, the means for connecting the magnetic field generator maycomprise two elongate arms with a connection point in an L shape, suchthat the generator is arranged substantially at the said connectionpoint, the first arm extends from the connection point to an adjoiningzone substantially on the surface A and the second arm extends from theconnection point to the surface B.

The connection means advantageously comprise a flexible soleplateconnecting the surfaces A and B.

A device of this kind may preferably be arranged close to the beads ofthe tire with a radial orientation D.

The distance L between the surface A and the surface B is advantageouslygreater than 20 mm, in order to give the device good sensitivity. In thecase of a tire for heavy vehicles, this distance may be in the order of30 mm.

The distance H, the spacing between the magnetic field generator and thepassive sensor, may be between 10 and 15 mm (in the free state, orbefore the device is mounted on the surface of the tire).

The magnetic field generator may be a permanent magnet or an assembly ofpermanent magnets. This generator may also be in the form of rubbermixes including magnetic particles. The permanent magnet or magnets arepreferably of the samarium-cobalt type, to ensure good performance athigh temperature.

The invention also relates to a tire carrying the above device forcounting rotations.

The device is preferably arranged on the inside surface of the tire. Thesurface A of the device may advantageously be arranged on the innersurface of one of the beads of the tire. The surface B is in this caseadvantageously arranged substantially radially outwards in relation tothe surface A. The distance E between the surface A and the radiallyinward end of the inner surface of the tire may be greater than 50 mm.

It is in fact found with this arrangement that the surface A has onlyvery small variations in profile on passing through the area of contact,while the surface B starts to have marked variations, greater than 1 mmin the case of tires for heavy vehicles. As a consequence, thevariations in the distance between the magnetic field generator and thepassive sensor have the same amplitude, and the device has excellentsensitivity.

The device according to the invention may also be arranged on the innersurface of the crown of the tire, such that the surfaces A and B areoriented substantially in the circumferential direction. Variations inthe relative positions of the surfaces A and B and of the magnetic fieldgenerator and the passive sensor are then associated with the flatteningof the crown of the tire when it passes through the area of contact.

The device according to the invention may, as another possibility, bearranged on the outside surface of the tire, at a radial height greaterthan the height of the rim hooks on which the tire has to be mounted, inorder to avoid any problem with mounting or during travel. In this case,maintenance of the device is facilitated. It is preferable in this caseto provide a device for protection, to avoid damage thereto.

The device according to the invention may furthermore include one ormore of the following features:

-   -   the device may include a signal interpreter which emits a pulse        each time it detects at least one cycle of signals emitted by        the passive sensor;    -   the interpreter of the device may include a comparator;    -   the interpreter of the device may include means for filtering        the signals emitted by the passive sensor;    -   the interpreter of the device may include means for amplifying        signals;    -   the device may include a counting means connected to the        interpreter;    -   the counting means of the device may include a counter which,        each time it receives a predetermined number of pulses from the        interpreter, transmits a pulse to a microprocessor including        means for storing the number of pulses received from the        counter;    -   the device may include a device for transmitting to the outside        data indicating the number of pulses stored by the        microprocessor;    -   the device may include an autonomous system of electrical power        supply to the electronic components, such as a battery;    -   the casing may include, in addition to the passive sensor, all        the means for electrical power supply and signal processing.

The invention also relates to a device for counting the rotations of anobject which undergoes deformation in the course of a rotation,comprising two parts subject to relative movement under the effect ofthe deformation, characterised in that the first part includes amagnetic field generator, while the second part includes a sensorsensitive to the said magnetic field.

The signals generated by the sensor as a result of the relative movementbetween the first part and the second part are thus associated with thedeformation of the object in the course of rotation and hence allow therotations to be counted easily.

The sensor may be in the form of a passive sensor, for example a simplecoil, which allows the need to supply electrical power to the sensor tobe avoided.

Similarly, the magnetic field generator may be a magnet, which alsoallows an electrical power supply at the generator to be avoided.

When one of the parts is connected to the object by an arm (or member),the latter may where appropriate have a certain resilience. Thisresilience on the one hand allows the risk of the arm breaking to beavoided, thus imparting good durability in functioning, and on the othercontributes to defining a reference position about which the partconcerned can be displaced. Even though this resilience can bring abouta slight phase offset between the relative movement of the sensor andthe magnetic field generator, on the one hand, and the deformation ofthe object, on the other, the frequency of the relative movement of thesensor and the magnetic field generator still corresponds to that of thedeformations generated by rotation of the object and hence can stillserve as the basis for counting these rotations.

In practice, the device may include an interpreter which emits a pulseeach time it detects at least one cycle of signals emitted by thesensor. The interpreter includes, for example, a comparator which allowsthe cycles to be determined particularly simply, and which consequentlyconsumes little power, and/or means for amplifying the signals, which isparticularly advantageous when a passive sensor is used.

The device may also comprise a counter which, each time it receives apredetermined number of pulses from the interpreter, transmits a pulseto a microprocessor which includes means for storing the number ofpulses received from the counter. Thus, the processor can operate foronly a minimal proportion of the time of use, which also allows theelectrical power consumption of the device to be reduced.

The device may furthermore include means for transmitting to the outsidedata indicating the number of pulses stored by microprocessor. Thenumber of pulses stored by the microprocessor which, it goes withoutsaying, indicates the number of rotations the object has performed, maythus be read off without making contact with another device.

The electrical power supply of the device may be provided by anautonomous system for the supply of electrical power to the electroniccomponents (for example a battery), in particular thanks to the lowpower consumption of the device thanks, for example, to some at least ofthe features above, which allows the device to be used even inenvironments in which no outside electrical power supply is available.

The invention will be more readily understood from reading thedescription below, which is given solely by way of example and whichrefers to the attached drawings, in which:

FIG. 1 is a side view of a first example embodiment of a deviceaccording to the invention;

FIG. 2 is a section CC as indicated in FIG. 1 of the first example of adevice according to the invention;

FIG. 3 is a plan view of the device in FIG. 1;

FIG. 4 shows the curve of the profile of the inner surface of a tire forthe azimuths 0° (opposite the area of contact) and 180° (centre of thearea of contact);

FIG. 5 is a centre line sectional view through a tire which is equipped,by way of illustration, with a plurality of devices according to theinvention arranged on its inside and outside surfaces;

FIG. 6 is a flow diagram of a device according to the invention;

FIG. 7 is a graph showing the signals emitted over time by the sensor ofa device according to the invention;

FIG. 8 is a side view of a second example embodiment of a deviceaccording to the invention;

FIG. 9 is a side view of a third example embodiment of a deviceaccording to the invention;

FIG. 10 is a side view of a fourth example embodiment of a deviceaccording to the invention; and

FIG. 11 is a side view of a fifth example embodiment of a deviceaccording to the invention.

FIGS. 1, 2 and 3 illustrate a side view, a section and a plan view of afirst example embodiment of a device 1 according to the invention. Thisdevice chiefly comprises a rigid casing 2, connection means 3, 4 and 7,and a sensor in the form of a passive sensor 5 and a magnet 6.

The casing 2 carries the passive sensor 5 and all the means 8 forelectrical power supply and signal processing.

The connection means include a soleplate 4, an element 3 for connectingthe casing 2 and an element 7 for connecting the magnet 6. The soleplate4 has a first face 41, which is intended to be permanently connected tothe surface of the tire, and a second face 42. This soleplate isadvantageously made of flexible rubber, so that it can follow thedeformations of the surface of the tire. The element 3 for connectingthe casing 2 allows the casing 2 and the soleplate 4 to be mademechanically integral with one another, such that the casing follows allthe displacements of the surface A, the zone of the soleplate adjacentto the connection element 3. This connection element 3 may also be madeof rubber material, but it must be substantially harder in order toguarantee its mechanical functions of holding and making integral. Themagnet 6 is embedded in the connection element 7.

The casing 2, the connection element 3 and the soleplate 4 are generallyU-shaped. The passive sensor 5 is arranged at the end of one of the twoarms of the U and the magnet 6 is arranged at the other end. These twoelements form the sensor of the device and are arranged opposite oneanother in a direction normal to the direction D connecting the surfacesA and B. The sensor is offset from the connection element 3 by a lengthL greater than 20 mm, and in the order of 30 mm for the tires of heavyvehicles. The magnet 6 and the passive sensor 5 are separated from oneanother by a distance H of between 10 and 15 mm in the free state. Theconnection element 7 positions the magnet 6 here, projecting from theend of the soleplate 4. It may also be embedded in the soleplate 4. Themagnet 6 will be able to follow all the displacements of the surface Bof the soleplate 4 and the surface of the adjoining tire.

The soleplate 4 also comprises a raised kerb 9 intended to receive andfix a hood for protecting the device when it is arranged on the surfaceof a tire, or during any retread operations thereon. A hood of this kindmay in particular be made of glass fibre reinforced plastics material.

FIG. 4 shows how the mid-line profile of the inner surface of a tire forheavy vehicles, 315/80 R 22.5, changes under given conditions of loadingand pressure for the azimuths 0° (opposite the area of contact—curve a)and 180° (centre of the area of contact—curve b).

It is found that there is virtually no variation in this profile, as itpasses through the area of contact, over the entire zone of beads of thetire, the zone close to the bead wire and bearing on the outer side ofthe tire against the rim hook. By contrast, on the radially outer sidethe spacing between the two profiles shown continues to increase.

Thus, a first favourable position for installation of the deviceaccording to the invention is with the surface A placed adjacent to theradially outer end of the zone of beads of the tire, so that thedisplacements of this surface A and thus of the casing and the passivesensor are very much reduced during travel. The surface B is in thiscase placed on the radially outer side in relation to the surface A.This is what is shown in FIG. 5 for the device 1.

With a distance L in the order of 30 mm between the centres of thesurfaces A and B, the variations in the profile and hence in thedistance between the passive sensor and the magnet as they pass throughthe area of contact are in the order of 1 mm.

The passive sensor 5 is advantageously a coil whereof the direction ofsensitivity (identical to the axis of the coil) is oriented in thedirection S, normal to the direction D. The magnet is also placed withits two poles oriented in the direction S. Any variation in the distancebetween the magnet and the coil will thus bring about a variation in themagnetic flux passing through the coil, and emits a signal proportionalto this variation, in accordance with Faraday's law and Lenz's law.

The signal generated by the coil is a periodic signal whereof thefrequency is equal to the frequency at which the tire rotates.

The stronger and faster the variations in the magnetic flux passingthrough the coil, the larger the signal generated by the coil: itsamplitude thus also depends on the speed of rotation of the coil. In aparticular embodiment, the signal is integrated to obtain the value ofthe field and eliminate the effect of the speed.

FIG. 6 is a flow chart of a particular embodiment of a device accordingto the invention. The device I is composed of a sensor with a passivesensor 5 (a coil having an impedance of 2.2 mH) and a magnet 6 of 100mm³, an amplifier 26 allowing the signals emitted by the passive sensorto be amplified if necessary, a filter 28 allowing interferencefrequencies to be suppressed and alternative external sources ofmagnetic field which create interference to be rejected (e.g. 50 Hz onthe mains network), and a comparator 30 which generates a rectangularpulse on each rotation of the tire. The amplifier 26, the filter 28 andthe comparator 30 form a signal interpreter. The device furthermorecomprises an N-bit counter 32 which stores the number of rotationsperformed by the tire and which, when it is full (every 2^(N)rotations), triggers a microprocessor 34 which increments its memory byone, for example. The counter 32 and the microprocessor 34 form acounting means. Finally, the device includes an output device 36 whichallows data indicating the number of pulses counted by themicroprocessor 34 to be transmitted to the outside. The electronicsinclude a battery 37.

The dimensions of the casing are approximately 50 mm×20 mm. The totalheight of the device is approximately 20 mm and its weight is in theorder of 20 g.

FIG. 7 shows the signals S1 and S2 supplied by the device according tothe invention over three wheel rotations under the following testconditions: inflation pressure 6 bar, load 35000 N and speed 10 km/h,with tire 315/80 R 22.5 (a tire for heavy vehicles). The signal S1 isthe output signal from the amplifier 26 and the signal S2 is the outputsignal from the comparator 30.

It is found that passing through the area of contact is shown, in theoutput from the amplifier 26, as a first peak of negative amplitudefollowed by a second of positive amplitude. At the output of theinterpreter, the signals comprise only a single pulse (S2),corresponding to each wheel rotation.

A test was carried out with a device of this kind glued to the innersurface of the bead of a tire mounted on the driving axle of aload-bearing vehicle MAN 14. The kilometres measured correspond to thekilometres covered, within 5%. The information on kilometres is obtainedfrom radio interrogation via a manual reader close to the tire, at therear of the vehicle. It is thus possible to transmit the results to thedriver's cab and to display them.

FIG. 5 shows, in addition to the position in the zone 10 which hasalready been described, two other possible positions in which a deviceaccording to the invention may be fixed to the surface of the tire 9.The first of these consists in fixing the device 1 to the outer surfaceof the tire in a zone 12, substantially at the same radial height on thetire side. A minimum height above the height of the rim hook 13 of thetire should be observed, to avoid any risk of damage on mounting orduring travel. It is preferable to add a device for protecting thedevice, to limit the risk of shock during travel. The advantage of thisposition is that it makes maintenance and monitoring of the deviceeasier.

Another position is below the crown of the tire, in a zone 16. The twoarms of the U of the device 1 are in this case arranged in acircumferential direction. The variations in distance between the magnetand the coil are linked to the flattening of the crown as it passesthrough the area of contact.

FIG. 8 shows a second embodiment of a device according to the invention.In this embodiment, the device 50 comprises, as above, a casing 52, asensor with a coil 53 and a magnet 54, an element 55 for connecting thecasing 52, an element 57 for connecting the magnet 54 and a soleplate56. The connection element 57 is arcuate and elongate in shape andextends from the surface of the soleplate 56 adjoining the surface B tothe surface of the casing 52 on the opposite side of the soleplate 56.As a consequence, any relative displacement of the surfaces A and B willbring about a relative displacement of the magnet and the coil which isdirected in a direction substantially perpendicular to the direction D.This device 50 has the advantage of allowing the surfaces A and B to beseparated from one another and thus to increase the amplitude ofrelative movements between the two elements of the sensor.

FIG. 9 shows a third embodiment of a device according to the invention.This device 60 comprises, as above, a casing 62, a sensor with a coil 63and a magnet 64, an element 65 for connecting the casing 62, an element67 for connecting the magnet 64 and a soleplate 66. The connectionelement 67 is arcuate and elongate in shape and extends from the surfaceof the soleplate 56 adjoining the surface B to a surface B′ arrangedsymmetrically in relation to the surface B on the other side of thecasing 62. The result is that the relative movements between the twoelements of the sensor are associated with the relative displacementsbetween the surfaces A and B and between A and B′. In certainconditions, for example when the device 60 is arranged on the innersurface of the tire below the crown, this allows the sensitivity of thedevice for counting the rotations of the tire to be increased. In thiscase, the device is preferably arranged with the direction D oriented inthe circumferential direction.

FIG. 10 shows a fourth embodiment of a device according to theinvention. In this embodiment, the device 70 comprises, as above, acasing 72, a sensor with a coil 73 and a magnetic field generator 74, anelement 75 for connecting the casing 72, an element 77 for connectingthe generator 74 and a soleplate 76. The element 77 for connecting thegenerator 74 separates these parts from the soleplate 76 by an averageheight of h, such that this generator is opposite the coil 73. Thegenerator of this device 70 is in the form of one or more magnets 74arranged opposite the coil 73, but in a direction parallel to thedirection D instead of perpendicular thereto as before.

As previously, the directions in which the magnets and the coil aresensitive are oriented in a manner normal to the direction D. Anyvariation in the relative positions of the magnets and the coil willalso bring about an electrical signal in the coil 73.

FIG. 11 shows a fifth embodiment of a device 80 according to theinvention. In this embodiment, the device 80 comprises, as above, acasing 82, a sensor with a coil 83 and a magnetic field generator 84, anelement 85 for connecting the casing 82, an element 87 for connectingthe generator 84 and a soleplate 86. The element 87 for connecting themagnetic field generator 84 is in the form of two elongate arms 88 and89 with a connection point 90, in the shape of an L. The generator 84 isarranged substantially at the connection point 90, the first arm extendsfrom the connection point 90 to a zone of the soleplate substantiallyadjoining the surface A and the second arm 89 extends from theconnection point 90 to the surface B. As a consequence, any relativedisplacement normal to the direction D of the surfaces A and B bringsabout a relative displacement of the passive sensor 83 and the magneticfield generator 84 substantially parallel to the direction D.

The embodiments described above have been given purely by way ofnon-restrictive examples, and may be the subject of any desirablemodifications without in so doing departing from the scope of theinvention.

1. A device for counting the rotations of an object which undergoesdeformation in the course of a rotation, comprising two parts subject torelative movement under the effect of the deformation, wherein the firstpart includes a magnetic field generator, while the second part includesa sensor sensitive to the magnetic field.
 2. The device according toclaim 1, wherein the sensor is a passive sensor.
 3. The device accordingto claim 1 wherein the magnetic field generator is a magnet.
 4. Thedevice according to claim 1, wherein one of the two parts is connectedto the object by a resilient member.
 5. The device according to claim 4,wherein the resilient member contributes to defining a referenceposition of the sensor in relation to the magnetic field generator. 6.The device according to claim 1, comprising an interpreter which emits apulse each time it detects at least one cycle of signals emitted by thesensor.
 6. The device according to claim 1, comprising an interpreterwhich emits a pulse each time it detects at least one cycle of signalsemitted by the sensor.
 7. The device according to claim 6, in which theinterpreter comprises a comparator.
 8. The device according to claim 6,in which the interpreter comprises means for amplifying signals.
 9. Thedevice according to claim 1, comprising a counter which, each time itreceives a predetermined number of pulses from the interpreter,transmits a pulse to a microprocessor including means for storing thenumber of pulses received from the counter.
 10. The device according toclaim 9, comprising means for transmitting to the outside dataindicating the number of pulses stored by the microprocessor.
 11. Thedevice according to claim 1, comprising an autonomous system ofelectrical power supply to the electrical components.
 12. A device forcounting the rotations of a tire on a vehicle using the deformationsthat the tire undergoes on each rotation of the wheel, comprising acasing, a sensor intended to generate an electrical signal on eachrotation of the wheel, and connection means intended to mechanicallyconnect the casing and the sensor to the surface of the tire, whereinthe sensor is in the form of a magnetic field generator, and a sensorsensitive to the magnetic field and in one piece with the casing,arranged opposite the magnetic field generator, wherein said connectionmeans mechanically connect the casing to a first surface A and themagnetic field generator to a second surface B, the two surfaces A and Bbeing substantially coplanar and offset from one another in a directionD by a given distance L, and wherein said electrical signal is producedby the relative displacements of said magnetic field generator and thesaid sensor sensitive to the magnetic field.
 13. The device according toclaim 12, in which the said sensor sensitive to the magnetic field is apassive sensor.
 14. The device according to claim 13, in which the saidpassive sensor is a loop of conductive wire.
 15. The device according toclaim 12, in which the connection means are such that a relativedisplacement normal to the direction D of the surfaces A and B bringsabout a relative displacement of the magnetic field generator and thepassive sensor substantially normal to the direction D.
 16. The deviceaccording to claim 15, in which the magnetic field generator and thesensor sensitive to the magnetic field are arranged in opposition in adirection normal to the direction D.
 17. The device according to claim16, in which the connection means, the casing and the sensor sensitiveto the magnetic field are arranged in an L shape.
 18. The deviceaccording to claim 17, in which the means for connecting the casing andthe sensor comprise a flexible soleplate having a first face intended tobe permanently connected to the surface of the tire and at least oneconnection element to which the casing is fixed at a given spacing inrelation to the soleplate.
 19. The device according to claim 18, inwhich the magnetic field generator is embedded in the soleplate.
 20. Thedevice according to claim 18, in which the casing, the element forconnecting the casing and the soleplate are generally U-shaped.
 21. Thedevice according to claim 16, in which the means for connecting themagnetic field generator are in the form of a resilient connectionelement of arcuate elongate shape extending in the direction D betweenthe surface B and a zone of the casing or element for connecting thecasing which is opposite the surface A, the magnetic field generator andthe sensor sensitive to the magnetic field being arranged in oppositionbetween the two ends of the said element for connecting the sensor. 22.The device according to claim 21, in which the connection means comprisea flexible soleplate connecting the surfaces A and B.
 23. The deviceaccording to claim 16, in which the means for connecting the magneticfield generator are in the form of a resilient connection element ofarcuate elongate shape extending in the direction D between the surfaceB and a surface B′ arranged beyond the surface A, the magnetic fieldgenerator and the sensor sensitive to the magnetic field being arrangedin opposition between the two ends of the said element for connectingthe magnetic field generator.
 24. The device according to claim 23, inwhich the connection means comprise a flexible soleplate connecting thesurfaces A, B and B′.
 25. The device according to claim 15, in which themagnetic field generator and the sensor sensitive to the magnetic fieldare arranged in opposition in a direction parallel to the direction D.26. The device according to claim 25, in which the connection meanscomprise an element for connecting the casing separating the said casingfrom the surface A by a given height and an element connecting themagnetic field generator separating the said magnetic field generatorfrom the surface B by a given height, and in which the sensor sensitiveto the magnetic field is in one piece with the casing and is arranged ata height in relation to the surface A which is substantially identicalto the height separating the magnetic field generator from the surfaceB.
 27. The device according to one of claim 12, in which the connectionmeans are such that a relative displacement normal to the direction D ofthe surfaces A and B brings about a relative displacement of the sensorsensitive to the magnetic field and the magnetic field generatorsubstantially parallel to the direction D.
 28. The device according toclaim 27, in which the means for connecting the magnetic field generatorcomprise two elongate arms with a connection point in an L shape, suchthat the generator is arranged substantially at the said connectionpoint, the first arm extends from the connection point to an adjoiningzone substantially on the surface A and the second arm extends from theconnection point to the surface B.
 29. The device according to claim 28,in which the connection means comprise a flexible soleplate connectingthe surfaces A and B.
 30. The device according to claim 12, in which thedistance L between the surface A and the surface B is greater than 20mm.
 31. The device according to claim 12, in which the distance Hseparating the magnetic field generator and the sensor sensitive to themagnetic field is between 10 and 15 mm in the free state.
 32. The deviceaccording to claim 12, in which the said magnetic field generator is arubber mix including magnetic particles.
 33. The device according toclaim 12, in which the said magnetic field generator is a permanentmagnet of the samarium-cobalt type.
 34. The device according to claim12, including an interpreter which emits a pulse each time it detects atleast one cycle of signals emitted by the sensor sensitive to themagnetic field.
 35. The device according to claim 34, in which theinterpreter includes a comparator.
 36. The device according to claim 34,in which the interpreter includes means for filtering the signalsemitted by the sensor sensitive to the magnetic field.
 37. The deviceaccording to claim 34, in which the interpreter includes means foramplifying signals.
 38. The device according to claim 34, including acounting means connected to the interpreter.
 39. The device according toclaim 38, in which the counting means is in the form of a counter which,each time it receives a predetermined number of pulses from theinterpreter, transmits a pulse to a microprocessor including means forstoring the number of pulses received from the counter.
 40. The deviceaccording to claim 39, including a device for transmitting to theoutside data indicating the number of pulses stored by themicroprocessor.
 41. The device according to claim 34, including anautonomous system of electrical power supply to the electroniccomponents, such as a battery.
 42. The device according to claim 12, inwhich the said casing is a printed circuit carrying, in addition to thesaid passive sensor, all the means for electrical power supply andsignal processing.
 43. A tire equipped with a device according toclaim
 1. 44. The tire according to claim 43, in which the connectionsurface A is arranged on the inner surface of one of the beads of thesaid tire.
 45. The tire according to claim 44, in which the surface B isarranged substantially radially outwards in relation to the said surfaceA.
 46. The tire according to claim 44, in which the said surface A isarranged at a spacing greater than 50 mm from the said radially inwardend of the inner surface of the tire.
 47. The tire according to claim43, in which the said device is arranged on the inner surface of thecrown of the said tire, and in which the surfaces A and B are orientedsubstantially in the circumferential direction.
 48. The tire accordingto claim 43, in which the said tire is intended to be mounted on a rimhaving rim hooks of given height and the said device is arranged on theouter surface of one of the sidewalls of the said tire, at a radialdistance greater than the height of the said rim hooks.
 49. A tireequipped with a device according to claim 12.